Passive and active RF components are integral to microwave and millimeter wave systems. Generally these components are designed based on the manufacturing methods and tolerances within the build process. Traditionally such processes include a computer numerically controlled (CNC) machine process or a die-cast process depending on the volume of the waveguide components to be made. These methods can suffer from multiple deficiencies such as the method of manufacturing being serial and not batch processed. For example, for CNC, the geometry of each machined part needs to be programmed into the machine for the build, and tolerances of the build depend on the tool cutters and temperature of the machine which can vary substantially. In addition, to achieve high resolution and accurate parts, the machine speed is often lowered and operated by a skilled machinist, increasing the overall cost. For die-cast processes, the resolution that can be achieved is often much coarser than the designs require, and unacceptable variation from die to die can reduce overall yield. Multiple part assemblies can also be complex and add to further errors in positional accuracy of pins, dowels, and features. The above drawbacks contribute to the high cost of passive and active microwave and millimeter wave components and modules, with recent years showing little improvement in the overall build process.
Planar circuits are alternative structures which can include microwave printed circuit boards with dielectrically loaded microstrip or coplanar structures. However, drawbacks for these circuits include insertion loss and lack of isolation between signal lines compromising signal integrity.
Another major drawback with both 3D machining and planar circuits is the lack of compactness or functional density. The machining of transmission lines such as waveguide channels are only performed in 2D surfaces in split waveguide formations. This limits the full 3D functionality where the active elements can only be placed in specific locations dictated by machining orientation. In planar circuits, a limitation of 3D multilayer parts includes poor thermal management due to high dielectric load between the interconnects and lack of inclusion of active elements such as integrated circuits in embedded architectures. Furthermore, planar multilayer circuits are heavy and can become a large burden for the overall system.
Millimeter-wave and THz waveguide structures made from cross-linked photoresist SU-8 are disclosed in Tian, Y, Shang, X & Lancaster, M J 2014, “Fabrication of multilayered SU8 structure for terahertz waveguide with ultralow transmission loss,” Journal of Micro/Nanolithography, MEMS, and MOEMS, vol 13, no. 1, 013002.,10.1117/1.JMM.13.1.013002, hereafter “Tian.” Such an approach is limited both in the process capability and the resulting structures. Metallizing a photoresist using methods such as electroless plating to create a sliced waveguide structure has many limitations.
A first limitation in the art is that a photoresist plastic such as SU-8 has very low thermal conductivity, so the electronic chips cannot dissipate the heat they generate through the plastic. A second is that a thin metal on plastic has a CTE mismatch preventing such structures from surviving the thermal cycles needed for consumer, industrial, and aerospace applications. A third is that such plastic structures and metallized plastic are not compatible with standard chip interconnect processes such as wirebonding. A fourth is that fusing metallized layers of plastic is difficult due to the inability for such structures to endure substantial mechanical compression without delamination, cracking, and peeling of the metal coatings on the plastic. A fifth limitation is the mechanical robustness of a stacked plastic part particularly when thin or small intricate features are required. A sixth limitation is the poor resolution offered after metallization of patterned plastic parts. In some cases, one might try electroplating rather than electroless plating on the plastic. As the parts are metal seeded and electroplated, current crowding effects unevenly electroplate the structure depending on the locations on the part exposed to the electroplating anode. This is even a larger problem for thicker electroplating in excess of 3 μm which would be required for mechanical strength of the parts. A seventh limitation is the accurate alignment of multi-stacking of parts due to the above (sixth limitation) over-plating of corners and edges. An eighth limitation is the overall number of stacks and their ordering and available features that can be created or used in a single monolithic plastic part. As each layer is added on top of a cured and exposed lower layer, it is chemically attacked throughout the fabrication process which will affect the interfaces between each layer causing delamination and poor adhesion. In addition, and more limiting in this eighth limitation, is that when attempting more than one layer of photoplastic in a monolithic construction, any added layer's photoexposure must fall inside the planar area of the previous layer's photoexposure so that the previous layer is not inadvertently photoexposed in an undesired region. A ninth limitation is the mechanical robustness of the metalized-plastic parts. For example, as the individual parts come together quite often mechanical screws are used to fixture parts and force the layers together for a no-gap connection. The metalized plastic parts cannot be tapped for a screw or pressed hard against each other for a firm contact. A tenth limitation is the lack of combined plastic (or non-conductor) and metal (or conductor) on the same integrated layer, which can be needed to isolate transmission lines from each other electrically and is an important attribute as the layers become more functionally capable. Thus, due to these limitations and more, there remains a need in the art for devices and methods that can achieve the above requirements while overcoming the limitations currently present in the art.